Michael's symptoms, mild but insistent, are familiar to anyone who knows the disease. When he isn't sitting on his hands or firmly gripping his wrist—almost hugging himself—an arm begins flapping like a flag in a rising wind. He crosses his legs, squirms and shifts in his seat, holds a knee as it bobs back and forth. After a little more than an hour sitting on a couch in his office, Michael J. Fox swallows a pale blue pill and waits for the nagging chorus of tics and tremors that are the calling card of Parkinson's disease (PD) to subside. "It's funny—my symptoms have been so in check lately," he says. "Usually, when things start to happen, I get up and walk around. But if I'm sitting still, you really notice it."

The actor is used to detailing the personal toll of his disease. It's part of the job. Five years ago, when he started his own charitable foundation to jump-start Parkinson's research, one third of the phone calls coming in were from concerned fans asking about his health. (A taped message reassured them he was doing fine.) That Fox found fame in the 1980s by playing young roles—he was in his mid-20s when his teenage characters in the TV show Family Ties and the blockbuster Back to the Future films turned him into a household name—heightens the surprise of seeing his familiar face accompany the progression of what is often perceived as an "old person's" disease. (In fact, about 40 percent of PD cases are diagnosed under age 60.)

At 44, Fox is fit, funny, and still preternaturally boyish (he was recently carded when attending an R-rated movie with his son, Sam), and he lives a busy life with his wife, actress Tracy Pollan, and their four children. (In addition to Sam, 16, they have twins Schuyler and Aquinnah, 10, and Esme, 4). He still acts occasionally—right now he's shooting several episodes of Boston Legal—but his symptoms limit his choice of roles. "I always play someone with PD or some other affliction," he says. "Parkinson's has affected my spontaneity. It also does a good job on your vanity. You spend a lot less time worrying about what you look like, because you really can't control it."

Parkinson's is a degenerative neurological disorder afflicting more than one million Americans, with some 60,000 new cases diagnosed each year. Fox was just 30 when his rare young-onset Parkinson's was discovered in 1991. By that time the cell damage had already begun—Parkinson's affects a small patch of neurons in the brain called the substantia nigra, which produces a chemical, dopamine, used by the body to control movement. As the disease advances, PD patients find that swallowing and speaking grow difficult, walking becomes unbalanced, and the limbs can freeze. The only treatment—oral replacement of the missing dopamine via a host of medication cocktails—disguises the symptoms but grows less effective with time.

For seven years Fox successfully hid his Parkinson's even as he remained in the public eye, timing his pills to calm the tremors when he performed. Finally, as those symptom-free windows closed, he reluctantly went public in 1998. He was certain it would end his career. Instead, it gave him a new and far more important calling—one that, as he often says, he can't take credit for choosing. "Left to my own devices," he says, lighting into a brief smile, "I wouldn't have been so inspiring."

After announcing his diagnosis, Fox was quickly approached by several Parkinson's groups, but he decided to go off on his own, founding an organization devoted solely to research and dedicated to the premise of finding a cure within the decade. Since 2000 the Michael J. Fox Foundation for Parkinson's Research has funneled $55 million into the hands of scientists. This is no celebrity charity sideline: it's the largest private institution in the PD research hunt today, and it has galvanized scientific work in the field by cutting the typical year-long grant-review process and investing strategically in the most promising research avenues.

"The key to the Fox Foundation is that they give out real money, at least $100,000 per project," says Jeffrey Kordower, M.D., a pioneer in the study of gene therapy for Parkinson's and a member of the foundation's scientific advisory board. "They've established a new business model for science."

Five years later, a lot has changed. The lean guerrilla operation Fox envisioned now boasts 33 employees, including two staff scientists, and a 34-member scientific advisory board. In 2004 the foundation granted $18 million, its biggest year yet. And Fox himself is different—he's the de facto public face of the disease, a role he plays with a mixture of wry charm and passionate advocacy, whether goading Congress for increased funding or talking to other PD sufferers.

"It changes you, this disease," Fox says. "And I've discovered a certain level of surrender to it, which isn't to say capitulation or allowing it to overwhelm me. But my accepting it, it saves me a whole lot of mental work. It is what it is what it is."

What it is these days is not so much a limitation as an integral part of his life. Fox's kids don't really think of him as an actor. Instead, they think of him as running a Parkinson's organization. Sam even helps out at the foundation sometimes, doing computer work.

The trick, Fox says, is not to let yourself be defined by the disease. "I was playing hockey a few years ago, and afterwards I was talking to Tracy and I said, 'It's this f---ing PD. I can't get in the corners and dig out the puck the way I used to. My legs start failing, and I start to fall down.' And she said, 'Mike, you're 42 years old. If you can't play hockey like you did when you were 16, that's why. It's not the Parkinson's.' It's like that in everything. You take it apart and say, 'What's PD? What's age? What's maturity?' "

It is Fox's unwillingness to let Parkinson's get him down that has made him such an effective fighter for his cause. He comes from a family of soldiers: his father, Bill, was a career officer in the Canadian army, and he recalls talking to his uncles about their exploits during World War II. "I'd say, 'I don't know how you could fight in a war—the minute someone fired a shot I'd throw down my gun and be gone. How do you not do that?' And they said, 'You just don't. You show up and do what you need to do.' To a much less dramatic degree, that's what I do. I don't have the luxury of saying this isn't something I'm going to participate in."

It's a duty, he says, that his father would have approved of. "I can rationalize that and say I didn't have the choice. But still, there's always a moment of surprise that I didn't throw down my gun and run away."